To measure the resistivity of earth formations, a typical logging tool is arranged to include a series of annular electrodes arranged on a central mandrel. The electrodes make it possible to send a measurement current and a focusing current into the formations and to measure at different points on the tool body the potential created by these currents. This logging technique is well known and an example of a logging tool arranged to make measurements at two investigation depths is described in the U.S. Pat. No. 3,772,589 issued to Scholberg.
A conventional electrode-type logging tool comprises a metallic mandrel covered with an insulating sleeve on which is fitted a stack of rings consisting alternately of conducting electrodes and insulating spacers. The wiring of the sonde, whose main function is to connect the electrodes respectively to the connectors of the sonde head, is placed in a space between the inside of the rings and the outside of the insulating sleeve. This space is not sealed and the borehole fluids such as drilling mud can penetrate therein by flowing between the rings. The wiring and the insulating sleeve are thus exposed to the electrically-conductive and corrosive drilling muds often found in the borehole. There is thus a possibility of insulation loss in the event of a very small cut in the insulation covering this wiring or in the sleeve. Moreover, it is difficult to clean the mid which has penetrated under the rings, thus involving snode dismantling problems and making practically impossible the checking of the insulation between electrodes after utilization in a borehole.